Multiband antennas for wireless voice and data communications are known. For example, common frequency bands for GSM services include GSM900 and GSM1800. A low band of frequencies in a multiband antenna may comprise a GSM900 band, which operates at 880-960 MHz. The low band may also include Digital Dividend spectrum, which operates at 790-862 MHz. Further, it may also cover the 700 MHz spectrum at 698-793 MHz. Ultra wide band antennas may cover all of these bands.
A high band of a multiband antenna may comprise a GSM1800 band, which operates in the frequency range of 1710-1880 MHZ. A high band may also include, for example, the UMTS band, which operates at 1920-2170 MHz. Additional bands may comprise LTE2.6, which operates at 2.5-2.7 GHz and WiMax, which operates at 3.4-3.8 GHz. Ultra wide band antennas may cover combinations of these bands.
When a dipole element is employed as a radiating element, it is common to design the dipole so that its first resonant frequency is in the desired frequency band. To achieve this, the dipole arms are about one quarter wavelength, and the two dipole arms together are about one half the wavelength of the desired band. These are commonly known as “half-wave” dipoles.
However, in multiband antennas, the radiation patterns for a lower frequency band can be distorted by resonances that develop in radiating elements that are designed to radiate at a higher frequency band, typically 2 to 3 times higher in frequency. For example, the GSM1800 band is approximately twice the frequency of the GSM900 band.
There are two modes of distortion that are typically seen, Common Mode resonance and Differential Mode resonance Common Mode (CM) resonance occurs when a portion of the higher band radiating element structure resonates as if it were a one quarter wave monopole at low band frequencies. For example, when the higher band radiating element comprises a dipole element coupled to a feed network with an associated matching circuit, the combination of a high band dipole arm and associated matching circuit may resonate at the low band frequency. This may cause undesirable distortion of low band radiating patterns.
For example, low band elements, in the absence of high band elements, may have a half power beam width (HPBW) of approximately 65 degrees. However, when high band elements are combined with low band elements on the same multi-band antenna, Common Mode resonance of the low band signal onto the high band elements may cause an undesirable broadening of the HPBW to 75-80 degrees.
Approaches for reducing CM resonance include adjusting the dimensions of a high band element to move the CM resonance up or down to move it out of band of the low band element. In one example, the high band radiators are effectively shortened in length at low band frequencies by including capacitive elements in the feed, thereby tuning the CM resonance to a higher frequency and out of band. See, for example, U.S. Provisional Application Ser. No. 61/987,791, the disclosure of which is incorporated by reference. While this approach is cost-effective, tuning the CM resonance above the low band often results in an undesirable broadening of the azimuth beamwidth of the low band pattern.
Another approach for reducing CM resonance is to increase the length of the stalk of a high band element by locating it in a “moat”. A hole is cut into the reflector around the vertical stalks of the radiating element. A conductive well is inserted into the hole and the stalk is extended to the bottom of the well. This lengthens the stalk, which lowers the resonance of the CM, allowing it to be moved out of band, while at the same time keeping the dipole arms approximately ¼ wavelength above the reflector. See, U.S. patent application Ser. No. 14/479,102, the disclosure of which is incorporated by reference. While this approach desirably tunes the CM resonance down and below the low band, it requires more space and entails extra complexity and manufacturing cost.